1. Field of the Invention
The present invention relates to a method for forming drainage grooves in the surface of a roll which is suitable for processing web-like materials, for example, paper, and a roll machined by said method.
2. Description of the Prior Art
Hereinafter, the procedure with regard to the making of paper will be discussed in more particular detail.
In modern paper making machines a water/fiber mixture having a solid content of about 2% is applied to an endlessly revolving screen. A large part of the water flows through the screen openings so that after a short time, the solid component has increased to about 50%. In the further course of the process, the paper web still of low strength is transferred to an endlessly revolving felt. For further dewatering, the paper web and felt are conducted jointly through the so-called wet press. Such presses are two-roll or multi-roll mills consisting, for example, of a stone roll with one or more pressing rolls. Whereas the stone roll is smooth on the paper side, the pressing rolls are usually grooved on the felt side. Due to the pressure in the roll nip, a further portion of the water is forced into the roll groove and on further rotating of the pressing rolls centrifuged out again. Grooved pressing rolls thus make a decisive contribution to dewatering and consequently, reduce the heating power to be applied in the dry section for further drying the paper.
For corrosion reasons, grooved rolls are made with a special steel covering.
Since this material is very tough and the necessary grooves relatively narrow and deep and although mechanical milling of the grooves is possible, it is very complicated and due to the tool consumption, very expensive.
An alternative method of applying a grooved special steel covering resides in wrapping a specially formed special steel band around a prepared roll body. The profile of this band or strip has a height of about 12 mm and in the region of the groove, a thickness corresponding to the web between the grooves.
At the bottom, the profile is as wide as the web plus groove. When the special steel strip is now coiled under tensile stress and application of pressure onto the roll body, a helically grooved roll surface is formed. Since the starting material is expensive, this method is also very expensive.
Moreover, if after a part of the roll has already been wrapped, the special steel strip tears, the already wrapped part must be freed of the strip again and the winding with a new strip started from the beginning.